Dose optimization in 125I permanent prostate seed implants using the Monte Carlo method
Introduction
Radiotherapy is a type of oncology treatment, where ionizing radiation is employed with the purpose of destroying the tumor cells, that now represents a preponderant role in cancer care since is expected that, approximately, 60% of patients diagnosed with cancer will make use of this type of technique during the course of their treatment, in one of its different forms. The ionizing radiation acts on the tumor tissue ionizing its atoms or molecules, with the consequent annihilation of the malignant cells. The ideal radiotherapy treatment would be one in which the doses of radiation were fully deposited in the tumor cells, with no dose being deposited in healthy tissues. In practice, however, ionizing radiation also causes damage to healthy tissues. Therefore, in order to provide high quality treatments for their patients, radiotherapy services make use of treatment planning systems, a tool that allows the calculation of the radiation doses that are going to be delivered to the tumor and determination of the procedures that have to be employed in order to spare the healthy tissues that surround the tumor region.
Among the treatment modalities for prostate cancer, permanent 125I radioactive seeds implant is one of the best alternatives in order to attain a local deposition of the radiation dose and a high degree of normal tissue sparing [1]. This prostate radiotherapy procedure makes use of a transrectal ultrasound probe and a grid display template. During the treatment planning phase, the ultrasound images are acquired to facilitate the evaluation of the position where each seed has to be positioned. The seeds, as well as the template used to guide the physician during the medical procedure, are illustrated in Fig. 1.
Several studies address the use of 125I seeds in prostate brachytherapy treatments. Generally, in these studies, dosimetric parameters that will serve as an indicator of the tumor volume coverage are calculated for one single seed. These parameters are the radial dose function, , the dose rate constant, Λ, and the anisotropy function, . However, there are some approximations generally performed by the treatment planning systems, as considering the patient tissues as being composed only by water and neglecting that each seed attenuates the photons emitted by the surrounding seeds, since they are constituted by high atomic number materials as silver and titanium. It is deemed that these approximations can alter the final dose distributions in the prostate and a lack of information related to computational simulations of the treatments itself, especially with the use of the Monte Carlo method, was observed in the literature.
In this work the Monte Carlo code MCNPX [2], [3] was used to simulate permanent prostate implants using 125I seeds in two different computational phantoms. The simulations consisted of two scenarios, the first taking in account the prostate with spherical geometry and the second one using the voxel phantom MAX, with voxels of . Both geometries represent a healthy prostate, with typical mass of 21.46 g. In order to determine if the dose delivered to the target volume meets the recommendations presented by the American Association on Physicist in Medicine TG 64 [4], the doses were calculated considering that a reduction of 0.84% in the total 125I radiation activity occurs every day (half life of 59.6 days).
After the Monte Carlo simulations, with further data processing using the software MATLAB [5], it was possible to create curves illustrating the dose distributions for some selected slices of the voxel phantom, which allowed a more realistic view of the organs and tissues submitted to high or low doses. The results were compared with some 125I implant quality indicators described in the literature, as and [6], [7].
Section snippets
Materials and methods
125I seeds used in radiotherapy have iodine adsorbed on the surface of a silver cylinder (), which is the center of the seed and also serves as a radiographic marker. The source encapsulation is usually composed by titanium [8]. In the 125I radioactive decay scheme, X-ray emissions are also observed: 22.1 keV (0.15 photons/disintegration) and 25.5 keV (0.04 photons/disintegration). These emissions are due to the interaction of the iodine photons with the silver cylinder
Dose rate constant
For the same type of seed, Table 1 compares the dose rate constant obtained by other authors with the value found in this work. It can be noticed that the results generated in this work, through the use of the Monte Carlo MCNPX code, are in good agreement with the data presented by the other authors, certifying the efficiency of the validation methodology.
Spherical and voxel phantoms
The simulation results for the MAX06 phantom and the water spherical phantom were presented taking in account initial seed activities of 0.27
Conclusions
The computational results obtained in this study clearly shows that the correct choice of the quantity of sources used in the implants as well as their location inside the treatment volume are fundamental in order to provide an effective treatment, since the underestimation of the absorbed doses may cause the recurrence of the tumor. Conversely, an overestimation of the absorbed doses may result in unnecessary damage to normal tissue, adjacent to the prostate.
With the computational simulations
Acknowledgements
The authors would like to thank the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), for their financial support.
References (18)
Monte Carlo simulation of X-ray and Gamma-ray photon transport on a graphics-processing unit
Comput. Phys. Comm.
(2010)Improved radial dose function estimation using current version MCNP Monte Carlo simulation: Model 6711 and ISC3500 125I brachytherapy sources
Appl. Radiat. Isot.
(2004)- et al.
Input files with ORNL-mathematical phantoms of the human body for MCNP-4B
Comput. Phys. Comm.
(2007) - A.P. Dicker, G.S. Merrick, F.M. Watermam, R.K. Valicenti, L.G. Gomella, Basic and Advanced Techniques in Prostate...
- D.B. Pelowitz (ed.), MCNPXTM Userʼs Manual Version 2.5.0, Los Alamos National Laboratory report LA-CP-05-0369, April...
- et al.
Permanent prostate seed implant brachytherapy, Report of the American Association of Physicists in Medicine Task Group No. 64
Med. Phys.
(1999) - MATLAB 7.10 for Windows Getting Started Guide, The Math Works, Inc.,...
- L.P.P. Amadei, Evolução Bioquímica através de Medidas Seriadas de Antígeno Prostático Específico (PSA) de Pacientes...
- et al.
Radioactive seed migration after prostate brachytherapy with Iodine-125 using loose seeds versus stranded seeds
Int. Braz. J. Urol.
(2009)
Cited by (1)
Study on the factors affecting the dose error of using I-125 seeds in the treatment of prostate cancer using the Monte Carlo method
2022, International Journal of Radiation Research